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UKBN? 9QJPyrrole is polymerised in the presence of ultrafine silica-coated magnetite particles; the resulting colloidal poly(pyrro1e)-silica-magnetite nanocomposites have a conducting polymer content of 75 mass% and exhibit superparamagnetism.

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An effective field model of an amorphous antiferromagnet is developed assuming a distribution of near neighbour distances and hence a distribution of effective field coefficients between interacting atom pairs. It is shown that such a material is characterized by a symmetrical distribution of the total effective field, which, in the absence of any spin correlation, is approximately Gaussian. The model predicts a precise critical or N&l temperature proportional to the square root of the average number of interacting near neighbours, and an unusual downward curvature of the reciprocal susceptibility variation with decreasing temperature (see Fig. 5 and 6). Two classes of amorphous antiferromagnet are examined: those with and those without magnetically isolated paramagnetic atoms. The limited experimental data support the former class.Es wird ein Modell des effektiven Feldes eines amorphen Antiferromagneten entwickelt, wobei eine Verteilung der Abstiinde der niichsten Nachbarn und damit eine Verteilung der effektiven Feldkoeffizienten zwischen wechselwirkenden Atompaaren angenommen wird. Es wird gezeigt, daB solches Material durch eine symmetrische Verteilung des totalen effektiven Feldes, das wegen Fehlen irgendeiner Spinkorrelation niiherungsweise gaussformig ist, charakterisiert wird. Das Modell sagt eine genaue kritische oder NBel-Temperatur, die proportional zur Quadratwurzel der mittleren Zahl der wechselwirkenden niichsten Nachbarn ist, und eine ungewohnliche, abwiirts gerichtete Kriimmung der rcziproken Suszeptibilitiitsiinderung mit fallender Temperatur voraus. Zwei Klassen amorpher Antiferromagneten werden untersucht ; solche mit und solche ohne magnetisch isolierte paramagnetische Atome. Die begrenzten experimentellen Daten unterstiitzen den ersten Fall. X I = s (1 -X I 171) Y E'ig. 5. The reciprocal susceptibilities versus temperature plotted (full curves) for an average of four near neighbours (2 = 4) with a = 1. The graphs are plotted in reduced units of temperature divided by the paramagnetic Nee1 temperature B and units of susceptibility divided by xn (given by equation (18)). The graph also shows calculated reciprocal susceptibility-temperature relationships (dashed curves) for a crystalline material with the same parameters as the amorphous material, assuming nearest neighbour interaction only

We have measured the magnetoresistance of stretched sheets of carbon nanotubes in temperatures ranging from 2 K to 300 K and in magnetic fields up to 9 T, oriented either perpendicular or parallel to the plane of the sheets. The samples have been partially aligned by post-fabrication stretching, such that the direction of stretching was either parallel or perpendicular to the direction of applied electric current. We have observed large differences between the magnetoresistance measured under the two field orientations, most pronounced at the lowest temperatures, highest fields, and for the laterally-aligned sample. Treatment of the sheets with nitric acid affects this anisotropy.We analyzed the results within the theoretical framework of weak and strong localization and concluded that the anisotropy bears the mark of a more unusual phenomenon, possibly magnetically-induced mechanical strain.